The present invention relates to a catalyst comprising a spinel which comprises copper (Cu), zinc (Zn) and aluminum (Al), to a method for producing the catalyst comprising a spinel which comprises copper (Cu), zinc (Zn) and aluminum (Al), as well as to the use of such a catalyst.
Published German Patent Application No. DE 43 01 470, discloses a CuAl.sub.2 O.sub.4 spinel wherein tin, lead and an oxide, a salt or an elemental form of an element of the second main or subsidiary group of the Periodic Table has been added, and subsequently calcined to form the final spinel. The spinel is used to decompose laughing gas (N.sub.2 O). However, this patent does not teach the use of this spinel as a catalyst for the catalytic oxidation of CO to CO.sub.2 and/or for the catalytic reduction of NO.sub.x, preferably of NO and NO.sub.2, especially at temperatures of several hundred degrees centigrade.
According to the present invention, a spinel is understood to be a material having the general chemical formula A.sub.a B.sub.b O.sub.4, which has, at least microscopically, a crystallographic or crystalline cubic lattice structure with face-centered oxygen ions and with tetrahedral or octahedral gaps. All the A atoms and up to 50% of the B atoms are disposed in the tetrahedral gaps and the remaining B atoms are disposed in the octahedral gaps. The term A atom or B atom refers here only to the crystallographic arrangement. From a material point of view, the A atoms, as well as the B atoms, can be different from one another.
For environmental reasons, the purification of gases, particularly of exhaust gases, which are produced by internal combustion engines for example, especially, by diesel engines and engines operating on a lean mixture, as well as the decrease in the CO content of exhaust gases and the denitrification of these gases, that is, the decomposition of nitrogen oxides (NO.sub.x), is a pressing problem.
Published European Patent Application No. EP 042 471 discloses a catalyst which is used for catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO.sub.2) and contains the metals copper (Cu), zinc (Zn) and aluminum (Al) as metal oxides. At least 60% of the copper present is bound to the aluminum oxide as a copper oxide/aluminum oxide spinel. Zinc oxide is disposed in the free spaces of the porous spinel in a proportion by weight of between 1% to 20%.
However, the catalyst disclosed in EP 042 471 is known only for its purifying action with regard to carbon monoxide, which is based on the catalytic oxidation of the carbon monoxide. Whether the catalyst is also suitable for the purification of gases containing NO.sub.x and/or hydrocarbons, as occur in the case of the above-mentioned internal combustion engines and also in incinerator power plants and the like, is not disclosed.
It is an object of the present invention to develop a catalyst that has good resistance to exhaust gases components, and also has a good purifying action, particularly for NO.sub.x. Moreover, it is a further object of the present invention to develop a method for the production of such a catalyst.
Pursuant to the present invention, these and other objects are accomplished with a catalyst wherein at least one element selected from the group consisting of palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium, lanthanum, cerium, vanadium, titanium, niobium, molybdenum, tungsten, salts thereof and oxides thereof is added to a copper oxide/zinc oxide/aluminum oxide spinel having the chemical formula EQU Cu.sub.A Zn.sub.B Al.sub.C O.sub.4
wherein A+B+C.ltoreq.3 and A&gt;0, B&gt;0 and C&gt;0.
Furthermore, these and other objects are also accomplished by providing a method for the production of such a catalyst comprising producing a spinel comprising copper, zinc and aluminum from copper oxide, zinc oxide and aluminum oxide, and incorporating in the spinel at least one element selected from the group consisting of palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium, lanthanum, cerium, vanadium, titanium, niobium, molybdenum, tungsten, salts thereof and oxides thereof.
As a result of the use of a Cu.sub.A Zn.sub.B Al.sub.C O.sub.4 spinel as a catalyst, wherein A+B+C.ltoreq.3, A&gt;0, B&gt;0, and C&gt;0, wherein at least one of palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium, and/or rare earth, such as lanthanum and cerium, vanadium, titanium, niobium, molybdenum, tungsten and/or salts thereof and/or oxides thereof have been added, the catalyst is largely resistant to exhaust gas components such as H.sub.2 O, NO.sub.x, CO.sub.2 and/or SO.sub.2 ; and the catalyst acts in an oxidizing manner on the hydrocarbons and in a catalytically reducing manner on the NO.sub.x. The reduction of NO.sub.x takes place in an oxygen-containing gas and in the presence of reducing agents, such as hydrocarbons. Preferably, hydrocarbons are present in a sufficient concentration in the exhaust gases of the internal combustion engine. In preferred cases, reduction rates of more than 60% can be achieved at temperatures above 300.degree. C.
According to the present invention, substoichiometric compounds in which Al.sub.2 O.sub.3 functions as a matrix and which have characteristic spinel lines in the X-ray spectrum, are also regarded as spinels. In such substoichiometric compositions, a spinel of the composition AAl.sub.2 O.sub.4 is present in an Al.sub.2 O.sub.3 matrix, so that an A.sub.(1-X) Al.sub.2 O.sub.4 stoichiometry results.
In a preferred embodiment, the catalyst is preferably a copper oxide/zinc oxide/aluminum oxide spinel having the chemical formula EQU Cu.sub.(1-B) Zn.sub.B Al.sub.2 O.sub.4
wherein 0&lt;B&lt;1.
In another preferred embodiment, the catalyst is a copper oxide/zinc oxide/aluminum oxide spinel having the chemical formula Cu.sub.0.5 Zn.sub.0.5 Al.sub.2 O.sub.4.
According to the present invention, the spinel is preferably used as a molded object or a split Villz billet, particularly a honeycomb.
According to the present invention, the spinel preferably comprises between 0.5% and 15% by weight cerium oxide, particularly between 1 and 8% by weight.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.